Subsurface scattering field
When Subsurface is 1, the Base color has no influence.
Still you can use  radius to extract different colors from the Subsurface color.

Why it doesn't work for you
You're using RGB Blue (0,0,1), so Red and Green cannot be extracted (as they're zero).

Example
I'm using some random Blue containing all three channels (RGB: 0.14, 0.19, 0.8):


Subsurface = 1 (so the base color has zero influence)
I can bring up plenty of Red from my Blue, using the Subsurface Radius.